1. Field of the Invention
The present invention relates to a speed change system for a work vehicle including: a pump swash plate operation mechanism configured to operate a swash plate of a variable capacity pump for continuously variable transmission device; a motor swash plate operation mechanism configured to operate a swash plate of a variable capacity motor for the continuously variable transmission device; a swash plate angle control unit for controlling the pump swash plate operation mechanism and the motor swash plate operation mechanism; and an operation position detector configured to detect an operation position of a speed change operation tool. A swash plate angle of the variable capacity pump is non-stepwise changed, and a swash plate angle of the variable capacity motor is stepwise changed.
2. Description of the Related Art
For such a speed change system for a work vehicle, Japanese Unexamined Patent Application Publication No. 6-17928 describes a system in which: by changing a swash plate angle of a hydraulic pump (variable capacity pump) of a hydrostatic continuously variable transmission device through the operation of a shift lever (speed change operation tool), a change gear ratio of the hydrostatic continuously variable transmission device is non-stepwise altered, and based on detection information from a pressure detecting unit for detecting a circuit pressure of a hydraulic circuit that communicates with the hydraulic pump and a hydraulic motor (variable capacity motor) of the hydrostatic continuously variable transmission device, a hydraulic motor is switched to a low-speed state when the circuit pressure of the hydraulic circuit is a predetermined pressure or more, and to a high-speed state when the circuit pressure is below the predetermined pressure. In this system, though the hydrostatic continuously variable transmission device are provided with the variable capacity pump and the variable capacity motor, the variable capacity motor is normally kept in a high-speed state, and cannot be changed to a low-speed state by a manual operation. Accordingly, even when the speed change operation tool is operated to an operation position on a low-speed side for performing a low-speed traveling of the work vehicle, the variable capacity motor remains in the high-speed state with a low torque. As a result, it becomes difficult for the work vehicle to smoothly travel in a large load situation, such as hill-climbing traveling and a working state in which a working device is connected. In addition, in a low-speed region in which the change gear ratio of the hydrostatic continuously variable transmission device is made large by operating the speed change operation tool to the low-speed side, an angle on a low-speed side exhibiting low hydraulic pressure transmission efficiency is more frequently used as the swash plate angle of the variable capacity pump, and thus a vehicle speed is unlikely to be stabilized during the low-speed traveling.
Accordingly, it may be proposed to provide a special operation tool that allows a manual switching operation of the variable capacity motor. However in this case, a number of operation tools mounted in the operation part should be increased, and it becomes necessary to perform a speed change operation by the speed change operation tool as well as an operation of the operation tool specialized for shifting the swash plate angle of the variable capacity motor, as speed change operation relative to the hydrostatic continuously variable transmission device. Therefore, there is a room for improvement in operability.
As a vehicle speed control system for the work vehicle, Japanese Unexamined Patent Application Publication No. 7-23610 (see paragraph [0012] and FIG. 1) describes the system including: a hydraulic cylinder for speed change configured to operate a trunnion axis of the hydrostatic continuously variable transmission device (corresponding to change gear ratio altering unit); a controller configured to control an actuation of the hydraulic cylinder for speed change (corresponding to vehicle speed control unit); a potentiometer type speed setter for detecting an operation amount of the shift lever; and a potentiometer type feedback sensor configured to detect an actual turning amount of the trunnion axis, wherein the controller controls the actuation of the hydraulic cylinder so that a detection value of the feedback sensor matches a set value by the speed setter. In the work vehicle, it is often the case that the change gear ratio of the continuously variable transmission device is set to a small change gear ratio (high-speed side) for the purpose of, for example, enhancing work efficiency. When the work vehicle is started in such a setting condition, a large load due to the small change gear ratio will be applied to the engine from a stage with a low engine rotational speed, and therefore, engine stall due to overload is likely to be generated.
Accordingly, in order to prevent generation of engine stall, an accelerator operation should be carefully performed. Especially in a case where the work vehicle is started in a situation where a load is large, such as in ascending a slope or in a working state with the working device connected, there is a notable tendency that engine stall occurs, and the accelerator operation should be further carefully performed. Therefore, there is a room for improvement in operability.
As a speed change control system for the work vehicle, Japanese Unexamined Patent Application Publication No. 7-23610 (see paragraph [0012], FIG. 1) describes the system including: the hydraulic cylinder for speed change configured to operate the trunnion axis of the hydrostatic continuously variable transmission device (corresponding to change gear ratio altering unit); the controller configured to control the actuation of the hydraulic cylinder for speed change (corresponding to vehicle speed control unit); the potentiometer type speed setter for detecting the operation amount of the shift lever; and the potentiometer type feedback sensor configured to detect the actual turning amount of the trunnion axis, wherein the controller controls the actuation of the hydraulic cylinder so that the detection value of the feedback sensor matches the set value by the speed setter. In this system, by a control actuation of the vehicle speed control unit, the actual turning amount of the trunnion axis of the hydrostatic continuously variable transmission device can be made correspond to the operation amount of the shift lever. However, it has been known in the hydrostatic continuously variable transmission device that, as the load becomes larger, a loss of a hydraulic pressure due to oil leakage becomes larger, resulting in reduction of power transmission efficiency. Therefore, even when the engine rotational speed is constant, and at the same time the actual turning amount of the trunnion axis of the hydrostatic continuously variable transmission device matches the operation amount of the shift lever maintained at a predetermined operation position, in a case where the load fluctuates during traveling, the power transmission efficiency in the hydrostatic continuously variable transmission device is then fluctuated, and thus the change gear ratio of the hydrostatic continuously variable transmission device becomes unstable and the vehicle speed changes to a large extent.
Accordingly, with respect to the work vehicle, such as a tractor, in which power from an engine whose speed is changed by the hydrostatic continuously variable transmission device is used for traveling and power from the engine whose speed is not changed by the hydrostatic continuously variable transmission device is used for work, if the configuration described in Japanese Unexamined Patent Application Publication No. 7-23610 is introduced, in the case where work, such as rotary tillage work, is performed in which the vehicle speed and an actuation speed of the working device are desired to be retained in a relationship in which they are proportional to the engine rotational speed, this relationship cannot be retained due to a large vehicle speed change caused by the fluctuation of the power transmission efficiency in the hydrostatic continuously variable transmission device, and thus it becomes difficult to make a working trail of the working device uniform with high accuracy. In other words, there is a room for improvement in enhancing work accuracy of rotary tillage work or the like.
It has been proposed that, in the work vehicle having the continuously variable transmission device, the change gear ratio of the continuously variable transmission device is divided into multiple speed change steps, and when the change gear ratio of the continuously variable transmission device is stepwise changed among the divided speed change steps, an integer value corresponding to the number of the divided speed change steps, for example, in a case where the change gear ratio of the continuously variable transmission device is divided into seven steps, a figure from “1”-“7” is displayed as the speed change step of the continuously variable transmission device, and when the change gear ratio of the continuously variable transmission device is changed in a continuous manner, unlike the stepwise change in which the integer alone is displayed, a decimal indication of a value from 1 to 7 is displayed so as to show the continuous change of the change gear ratio of the continuously variable transmission device. Regarding this technique, for example, Japanese Unexamined Patent Application Publication No. 2006-70943 can be referred to. However, it describes a technique in which the change gear ratio of the continuously variable transmission device is decimally expressed merely based on the integer value corresponding to the the divided speed change step, and therefore, such a technique does not have enough meaning to introduce to the work vehicle, and it requires further elaboration and creation to introduce such a technique.